Abstract: Subcortical pathology is a common feature in aging, Alzheimer's disease and vascular dementia but has been challenging to study with micron resolution in vivo. Optical methods such as two-photon microscopy image the superficial cortex at the micron-scale, but the resolution of these conventional microscopic methods degrades rapidly beyond 600 microns imaging depth. Standard whole-brain magnetic resonance imaging (MRI) methods do not yet provide cellular-level resolution and are expensive. Thus, there is a pressing need for methods to assess deep cortical and subcortical perfusion and cellular injury at the microscopic level, thus bridging the gap between existing superficial optical microscopy and macroscopic imaging. This proposal will develop, validate, and demonstrate advanced optical microscopy methods for longitudinal imaging of subcortical structures in the mouse brain using 2200 nm Optical Coherence Microscopy. 2200 nm imaging, in which tissue scattering is reduced by 2.5 and 1.5 compared to 1300 nm and 1700 nm, respectively, will enhance the delivery of ballistic (as opposed to multiply-scattered) photons to the focal spot, and enhance the proportion of photons backscattered from the focus that are detected without further scattering. Both of these benefits will substantially improve the signal localization, spatial resolution and signal-to background ratio when imaging deep in the brain. These methods will push penetration depths further into the living mouse brain, imaging subcortical structures (i.e. hippocampal proper and dentate gyrus) and pathology at higher resolutions than were previously possible.